TAMPA — A new government report says Boeing Inc. is making good progress manufacturing the Air Force's next-generation aerial-refueling jet but warns of the possibility of unforeseen delays in such a complex program.

The report last week by the General Accountability Office says development of the KC-46 refueling aircraft is especially important because the nation's current fleet of Eisenhower-era KC-135s tankers requires extensive maintenance to keep flying.

MacDill Air Force Base is the home of 16 KC-135 "Stratotankers," and the Air Force's long-range plans call for the base to get eight additional KC-135 tankers in 2018 once the KC-46 begins rolling off the Boeing assembly line.

"The (KC-135) force is now over 50 years old on average and costs increasingly more to maintain and support, with additional concerns that age-related problems could potentially ground the fleet," the GAO report says.

The additional KC-135s slated for MacDill would come from McConnell Air Force Base in Kansas, which is to receive the first deliveries of the new KC-46s in 2017 and would no longer need the Stratotankers, officials say.

So a delay in the $52 billion KC-46 program may delay MacDill adding those eight tankers.

"The next 12 months will be challenging as the program must complete software development, verify that the software works as intended, finalize developmental flight test planning and begin test flights," GAO says.

The Pentagon has previously said the KC-46 may be delayed up to a year. The GAO report provided no estimate of additional delays. Boeing maintains it is on schedule.

GAO said it was concerned that Boeing is burning through a cash reserve fund meant to cover the costs of unexpected development costs.

That fund, initially $354 million, is expected to be depleted by September.

"We raised concerns about the high rate at which the management reserves were being used because doing so early in a program is often an indicator of future contract performance," the GAO says.

Boeing is scheduled to deliver 18 operational aircraft by August 2017. The company is then scheduled to deliver 15 aircraft per year with a final six provided in 2027, GAO says.

The KC-46 is expected to be a quantum leap over the KC-135. For one thing, unlike the KC-135, the new tanker will have "protection from nuclear, infrared heat-seeking missiles and biomedical threats," the GAO says.

"The KC-135 is restricted in tactical missions and does not have sufficient defense systems relative to the KC-46," the GAO says.

But such a complex aircraft poses design and manufacturing challenges.

For instance, the GAO notes, the KC-46 requires 15.8 million lines of computer code. But Boeing, it says, is reusing existing software and computer code which "has helped reduce risks associated with software development."

Still, "software verification testing has not yet started and software problem reports are increasing," the GAO says.

Boeing has started manufacturing four development KC-46s and will conduct test flights five to six days a week to maintain an aggressive test schedule, the GAO says.

Boeing acknowledges that "maintaining the program's flight test pace is among the program's greatest risks," the report says.

The first KC-46 test flight is scheduled early next year. MacDill is not yet scheduled to receive any of the new tankers. The Air Force has a fleet of more than 400 KC-135s.